1. Field of the Invention
The present invention relates to a transformer suitable for use in a semiconductor device, and particularly, to a transformer for use in a high frequency semiconductor device inside a monolithic microwave integrated circuit (MMIC), and the like, that is used in a fast radio transmission system, and so forth.
2. Description of the Related Art
There has been described a conventional transformer in a literature, the college text, xe2x80x9cElectromagneticsxe2x80x9d, by Koichi Shimoda and Soshin Ckikazumi, pp. 214, 216. FIG. 1 is a view broadly showing a conventional transformer described in the literature mentioned above. Referring to the figure, the conventional transformer is described hereinafter. Reference numeral 21 denotes a primary coil, 22 a secondary coil, and 23 a core. The primary coil 21 and secondary coil 22 are wound round the core 23.
Now, the operation is described hereinafter. Flow of alternating current in the primary coil 21 causes magnetic fluxes to be induced in the core 23, whereupon an electromotive force is generated in the secondary coil 22 by the agency of the magnetic fluxes induced.
It is an object of the invention to provide a transformer that can be applied to a high frequency semiconductor device, and the like. The transformer can be formed without the use of a conventional core and coils by forming at least two spiral inductors selected from a plurality of spiral inductors on a semiconductor substrate so as to overlap each other substantially in the vertical direction with an interlayer insulator interposed therebetween.
According to the invention, a transformer can be implemented without the use of a core and coils by forming a plurality of spiral inductors on a semiconductor substrate, and by forming at least two spiral inductors selected from the plurality of the spiral inductors so as to overlap each other substantially in the vertical direction to the substrate with an interlayer insulator interposed therebetween such that at least the two spiral inductors are insulated from each other in terms of d.c., but continuous with each other in terms of a high frequency wave.
Further, the transformer with the features described above may be formed wherein at least the two spiral inductors selected from the plurality of the spiral inductors are formed in the shape of a rectangle such that the rectangles overlap each other along the longer sides thereof, so that advantageous effects of mutual inductance can be enhanced by enlarging an area of overlapping portions without enlarging areas of elements.